Ultra-high purity (UHP) gases are preferred for the manufacture of semiconductor devices, laboratory research, mass spectrometer instruments and other industries and applications. UHP gases are typically defined as at least 99.9999999% pure gas by volume.
There are several methods of producing UHP gases. The method used is generally determined by the desired flow rate of gas. Small flow rates of UHP gas are most typically utilized in a single point of use (POU) such as a single laboratory test station or a single process tool such as a chemical vapor deposition (CVD) semiconductor processing tool. For POU applications and other similar, small flow rate applications, the preferred methods are either a cold reactive purification process, or a heated getter purifier process, primarily due to the small scale production possible with these methods.
A cold reactive purification process typically utilizes a reactive metal, metal alloy or polymer resin based purification material. The cold reactive purification process exposes a specific, reactive purification material to an impure gas to be purified. Typically, this method requires, specially conditioned purification material to "activate" the purification material, and then forcing the impure gas through the purification material under pressure. The purification material chemically or physically bonds with the impurities in the impure gas such that purified gas flows out of the enclosure.
FIG. 1 is a flow chart illustrating a prior art cold reactive purification process 10 utilizing porous, reduced nickel catalyst pellets as the purification material. First in an operation 14, an impure gas, such as argon, containing trace impurities of oxygen, is caused to flow through the porous nickel pellets under pressure. The trace oxygen impurities react with the nickel metal to form nickel oxide. Other impurities can also react with the nickel. The purified gas is then used for the intended purpose (e.g. a semiconductor fabrication process) in an operation 16. When the nickel has been substantially oxidized by the process 10 then oxygen and other impurities will no longer be effectively removed from the gas.
The cold reactive purification material is described as having reached "purification capacity" when the cold reactive purification material has reacted with the impurities to a point wherein the purification material can no longer remove the impurities from the impure gas to the required performance level. Using the above example of nickel, the nickel has reached purification capacity when the nickel can no longer remove the trace oxygen from the impure gas such that the resulting gas is no longer at least 99.999999% pure. When the cold reactive purification material has reached purification capacity for a given impurity, the purification material must be replaced or, if possible, reactivated.
The "total capacity" of an impurity is the amount of impurity required wherein substantially all of the purification material has been consumed or otherwise reacted by the impurity and therefore the purification material can no longer react with any impurities.
The total capacity for a given impurity is typically much greater than the purification capacity. Increased purification capacity for impurities is typically obtained by utilizing a larger volume of purification material.
Cold reactive purification material processes have the advantage of being very simple to operate in that they are typically "activated" at the factory and operate in the field without requiring a control or monitoring system. Cold reactive purification processes have the disadvantage of being capable of only removing a very limited number of types of impurities and then only having a small capacity for the limited number of types of impurities. Cold reactive purification process purifiers have been known to overheat and even cause fires if exposed to too great of a concentration of impurities at one time.
Heated getter purification materials, hereafter referred to as getter, are typically alloys and mixtures of Zr, Ti, Nb, Ta, V and other materials. A heated getter process is one where an impure gas to be purified is exposed to a quantity of getter material maintained at an appropriate temperature.
FIG. 2 illustrates a prior art heated getter process 30. In operation 31, the impure gas flows in the inlet in a first end of the purifier. Then, in an operation 32, the impure gas is preheated to operating temperature. Next, in an operation 34, the heated getter chemically bonds with impurities such as CO.sub.2, H.sub.2 O, CH.sub.4, CO, O.sub.2, N.sub.2, and other impurities in the impure gas. In an operation 36, the purified gas is cooled to near ambient temperatures in a heat exchanger process. At near ambient temperatures, getters have a large capacity for H.sub.2. An optional operation 38 substantially removes residual H.sub.2, and other impurities, utilizing a quantity of near ambient temperature getter. In an operation 39, the purified gas flows out the outlet in the second end of the purifier. The purified gas is then used for the intended purpose (e.g. a semiconductor fabrication process) in an operation 40.
A getter has reached purification capacity when the getter can no longer remove impurities to the required performance level. When a getter material has reached purification capacity for a given impurity, the getter must be replaced.
The total capacity of an impurity is the amount of impurity required wherein substantially all of the getter has been consumed or otherwise reacted by the impurity. The total capacity for a given impurity is typically much greater than the purification capacity for that impurity. Increased purification capacity for impurities is typically obtained by utilizing an increased volume of getter or heating the getter to a higher temperature.
A heated getter process has the advantage over the cold reactive purification material process of removing as much as fifty times the quantity of a given impurity for an equivalent quantity of purification material. The heated getter process also removes several types of impurities where the cold reactive purification material process typically removes only one or two types of impurities. This reduces the overall operation and maintenance cost of these systems.
Disadvantages of the heated getter process include: increased cost of the purification material alloy; requirement of a heat source which then requires a method of controlling the heat source such as a control system; and a requirement to cool the purified gas after purification. There also exists a potential of danger for personnel around heated getter purifiers since they typically operate at 300.degree. C. or higher and personnel can be burned on contact. Heated getter purifiers have also been known to cause fires if, while at operating temperature, the heated getter is exposed to too great of a concentration of impurities at one time. Due to the requirements of a heat source, a control system and gas cooling system, heated getter purifiers are typically built for medium and larger scale applications.
Most POU applications require small scale control and monitoring in addition to purification. These control, monitoring and purification assemblies are known as a "gas stick." FIG. 3 is a schematic diagram of a typical prior art gas stick 50. The gas stick 50 includes a pressure regulator 52 to control gas pressure, an inlet valve 54 to control inlet gas flow, a heated getter purifier 56 to purify the gas, a purifier outlet isolation valve 58 to isolate the purifier, a mass flow controller or mass flowmeter 60 to control or monitor the gas flowrate, a check valve 62 to control reverse gas flow, a pressure transducer or pressure gauge 64 to monitor system pressure, and an outlet filter 66 to remove particles from the gas flow.
Gas sticks are typically assembled utilizing two sealing methods or combinations thereof. Metal to metal sealing surfaces with crushable seals 68 are typically utilized for components which are replaceable. Welded connections 70 are typically utilized for components not typically replaceable. Both assembly methods are expensive and difficult to properly utilize.
The gas sticks grow more complicated as purification, control and instrumentation requirements increase. More complicated gas sticks become larger and more difficult to use in the limited space in a typical POU. Recently, gas sticks have evolved to include gas mixing, gas purging, and gas source selection in addition to the previous functions and components listed above.
The increased size and complexity of gas sticks has driven the development of a modular gas stick. An example of a modular gas stick specification can be found in the Specification For Surface Mount Interface of Gas Distribution Components, SEMI draft doc. #2787, 1998, incorporated herein by reference.
The specification for Surface Mount Interface (SMI) defines a modular interface to gas stick type components. FIG. 4A illustrates an example of a modular gas stick substrate 80 and FIG. 4B illustrates a modular gas stick component base 100. The modular gas stick substrate 80 includes machined passages 82 for gas flow, for example, from a component station 84 to component station 94. Each component station 84, 94, 95, 96, 97, 98 and 99 includes an inlet port 86 and an outlet port 88 which corresponds to inlet port 102 and outlet port 104 in each modular gas stick component base 100. A plurality of component types such as valves, mass flowmeters, and pressure transducers may be manufactured with a modular gas stick base 100. Each modular gas stick component base 100 seals to a common modular gas stick substrate 80. Modular gas stick substrates 80 may be utilized in a single or a multiple gas passage 82 design.
FIG. 5 illustrates several typical components installed on a modular gas stick substrate 80, namely, a first component 106, a second component 108, and a third component 110. Gas flows into an inlet 90 of the modular gas stick substrate 80, through a passage 82 in the modular gas stick substrate 80 to a first component station 84, through inlet port 86 in substrate 80, through an inlet 102 of the first component 106 through the first component 106 to an outlet 104, through an outlet port 88 of substrate 80. From the first component 106, gas flows into passage 82 in the modular gas stick substrate 80, to a second component 108. The flow continues from component station to component station until the gas reaches an outlet 92 of the modular gas stick substrate 80.
The modular gas sticks offer two important advantages over traditional gas sticks: First, a modular gas stick assembly and maintenance is faster, simpler and easier. Second, a modular gas stick is very compact in size. Modular gas sticks are more compact by following strict size and shape limitations on all components. As an example, SMI specifies the size of components other than valves and MFC/MFM, such as purifiers, to be confined to an envelope defined as a base 38.15 mm in width, 38.15 mm in depth, and a height of 180 mm.
The simple, ambient temperature operation of the cold reactive purification material process is utilized and works well in modular gas stick applications. However, heated getter processes have not been used with modular gas sticks. This is due, in part, to temperature limitations of the gas stick substrates which typically require temperatures of 40.degree. C. or lower. Many heated getter purifiers heat the getter material to temperatures between 200.degree. C. and 400.degree. C., and as such, would be incompatible with modular gas sticks implementation. Furthermore, heated getter purification is generally considered a larger, less compact scale application than those associated with modular gas stick systems.